Stress bimorph MEMS switches and methods of making same

ABSTRACT

A micro-electromechanical system (MEMS) switch formed on a substrate, the switch comprising a transmission line formed on the substrate, a substrate electrostatic plate formed on the substrate, and an actuating portion. The actuating portion comprises a cantilever anchor formed on the substrate and a cantilevered actuator arm extending from the cantilever anchor. Attraction of the actuator arm toward the substrate brings an electrical contact into engagement with the portions of the transmission line separated by a gap, thus bridging the transmission line gap and closing the circuit. In order to maximize electrical isolation between the transmission line and the electrical contact in an OFF-state while maintaining a low actuation voltage, the actuator arm is bent such that the minimum separation distance between the transmission line and the electrical contact is equal to or greater than the maximum separation distance between the substrate electrostatic plate and arm electrostatic plate.

BACKGROUND

[0001] 1. Field

[0002] The present invention relates to micro-electromechanical systems(MEMS) and, in particular, to a micromachined electromechanical radiofrequency (RF) switch that can preferably function over a range ofsignal frequencies from 0 Hz to approximately 100 GHz.

[0003] 2. Description of Related Art

[0004] MEMS (micro-electromechanical system) switches have a widevariety of uses in both military and commercial applications. Forexample, electrostatically actuated micro-electromechanical switches canconduct RF current in applications involving the use of antenna phaseshifters, in the tuning of reconfigurable antenna elements, and in thefabrication of tunable filters.

[0005] A representative example of a prior art MEMS switch is disclosedin Yao, U.S. Pat. No. 5,578,976, issued Nov. 26, 1996. Typically, thistype of MEMS switch is fabricated on a semi-insulating substrate with asuspended micro-beam element as a cantilevered actuator arm. Thecantilever arm is coupled to the substrate and extends parallel to thesubstrate, projecting over a ground line and a gapped signal line formedby metal microstrips on the substrate. A metal contact, preferablycomprising a metal that does not easily oxidize, such as platinum, gold,or gold palladium, is formed on the bottom of the cantilever arm remotefrom the fixed end of the beam and positioned above and facing the gapin the signal line. A portion of the cantilever arm and an armelectrostatic plate located thereon reside above the ground line on thesubstrate. When a voltage is applied to the arm electrostatic plate,electrostatic forces attract the arm electrostatic plate, and thus thecantilever arm, toward the ground line on the substrate, bringing themetal contact into engagement with the separate portions of the gappedsignal line, and thereby bridging the gap in the signal line.

[0006] Another example of an RF MEMS switch utilizing a cantileveractuator arm is disclosed in Loo et al., U.S. Pat. No. 6,046,659, issuedApr. 4, 2000. In Loo et al., the cantilever actuator arm comprises amultiple layer structure containing the arm electrostatic platesurrounded by insulating layers. As in Yao, the RF MEMS switch disclosedby Loo et al. provides a metal contact that bridges a gap between twoportions of an RF signal line, when the switch is closed. Both Yao andLoo et al. disclose that the cantilever actuator arm is generallydisposed parallel to the surface of the substrate when the RF MEMSswitch is in the open position. Thus, the distance between the metalcontact and the RF signal line when the RF MEMS switch is in the openposition is limited to the distance between the cantilever actuator armand the substrate along nearly the entire length of the cantileveractuator arm.

[0007] RF MEMS switches provide several advantages over conventional RFswitches which use transistors. These advantages include lower insertionloss, improved electrical isolation over a broad frequency range, andlower power consumption. Since this type of switch is fabricated usingexisting integrated circuit (IC) processing technologies, productioncosts are relatively low. Thus, RF MEMS switches manufactured usingmicromachining techniques have advantages over conventionaltransistor-based RF switches because the MEMS switches function likemacroscopic mechanical switches, but without the associated bulk andrelatively high cost.

[0008] However, integrated RF MEMS switches are difficult to implement.Due to the proximity of the electrical contact formed on the cantileverarm to the signal line formed on the substrate, these switches tend toexhibit poor electrical isolation at high frequencies. In the RF regime,close proximity of the electrical contact and the signal line allowsparasitic capacitive coupling between the contact and signal line whenthe switch is in the OFF-state, creating an AC leakage path for highfrequency signals. These losses, which increase with signal frequency,limit the use of MEMS switches in high frequency applications.

[0009] Capacitive coupling may be reduced by increasing the separationdistance between the signal line formed on the substrate and the metalcontact formed on the cantilever arm. However, in the MEMS switchdescribed above, there is a design tradeoff between the OFF-statecapacitance and the switch actuation voltage. This tradeoff can beexpressed mathematically. The OFF-state capacitance of the switch isgiven by the relation: $\begin{matrix}{C_{OFF} = \frac{ɛ\quad ɛ_{0}A}{d}} & (1)\end{matrix}$

[0010] where A is the area of overlap between the contact and the signalline, d is the distance between the contact and the signal line, e₀ isthe permittivity of free space and e is the dielectric constant of thematerial between the contact and the signal line.

[0011] The actuation voltage of a cantilever beam in a switch asdescribed above can be approximated by: $\begin{matrix}{V_{S}^{1} \approx \sqrt{\frac{18\quad E\quad I\quad d^{3}}{5\quad ɛ\quad ɛ_{0}L^{4}w}}} & (2)\end{matrix}$

[0012] where E is Young's modulus of the beam material, I is the momentof inertia of the beam cross-section, and L and w are the length andwidth of the cantilever beam, respectively. For a cantilever beam with auniform width w, and a thickness t, the moment of inertia is given by:$\begin{matrix}{I = \frac{t^{3}w}{12}} & (3)\end{matrix}$

[0013] and V_(S) can be simplified to: $\begin{matrix}{V_{S} = \sqrt{\frac{3{E\left( {d\quad t} \right)}^{3}}{10\quad ɛ\quad ɛ_{0}L^{4}}}} & (4)\end{matrix}$

[0014] Combining the above expressions (1) and (4) yields

C _(OFF) ∝V _(S) ^(−2/3)  (5)

[0015] Thus, in the RF MEMS switches of the type described above,increasing the separation distance between the signal line formed on thesubstrate and the electrical contact formed on the cantilever arm alsoincreases the voltage required to affect electrostatic actuation of theswitch, because the separation distance between the signal line and thecontact is also the separation distance between the arm electrostaticplate and the ground line. The energy that must be moved through theswitch control in order to activate the switch, and thus the energydissipated by the switch, is a function of the actuation voltage.Therefore, in order to minimize the energy dissipated by the RF MEMSswitch, it is desirable to minimize the actuation voltage of the switch.

[0016] Another problem with the conventional cantilever switch describedabove stems from the methods used to manufacture the switch. Apolycrystalline silicon (or polysilicon) cantilever beam can befabricated by first oxidizing a silicon substrate to provide asacrificial layer, then depositing and patterning a layer of polysiliconinto a long, narrow bar directly over the silicon dioxide. The beam isthen separated from the sacrificial silicon dioxide layer by applicationof a release agent comprising a hydrofluoric acid solution, whichdissolves the sacrificial layer and results in a free-standingpolysilicon beam spaced apart from the substrate. The substrate isimmersed in the release agent for a duration sufficient to result inrelease of the beam. One problem with the use of this release processfor a beam in relatively close proximity to the substrate is thatsurface tension forces exerted by the release agent tend to pull thebeam toward the substrate as the device is immersed in and pulled out ofthe solutions. This can cause the beam to stick to the substrate duringdrying, a phenomenon known as stiction.

[0017] In view of the foregoing, there is a need for amicro-electromechanical switch having improved electrical isolation andimproved manufacturability, without requiring a corresponding increasein actuation voltage.

SUMMARY

[0018] Embodiments of electromechanical switches according to thepresent invention minimize the OFF-state capacitance of theelectrostatically actuatable micro-electromechanical switch formed on asubstrate, without a corresponding increase in the voltage required toactuate the switch. Embodiments of the present invention achieveminimization of the OFF-state capacitance by utilizing an actuator armbent such that the minimum separation distance between an electricalcontact formed on the actuator arm and a transmission line formed on thesubstrate is equal to or greater than the maximum separation distancebetween a substrate electrostatic plate formed on the substrate and anarm electrostatic plate formed on the actuator arm. The bilaminarcantilever structure of the preferred embodiments enable a largeseparation (up to approximately 300 micrometers) to be achieved betweenthe transmission line formed on the substrate and the electrical contactformed on the actuator arm, while maintaining a very low actuationvoltage (approximately 20 V). This large separation can be used toreduce the capacitance of the RF MEMS switch in the OFF state, thusproviding high isolation at high frequencies.

[0019] The desired minimization of the OFF-state capacitance is achievedwithout a corresponding increase in the actuation voltage by forming thearm electrostatic plate at a point on the actuator arm that allows thedistance between the arm electrostatic plate, formed on the actuatorarm, and the substrate electrostatic plate, formed on the substrate, tobe precisely and repeatably controlled, thus allowing the actuationvoltage to be correspondingly controlled.

[0020] The tendency of the beam to stick to the substrate during dryingis reduced by forming the bend in the actuator arm through thegeneration of unbalanced residual stresses in either the polycrystallinesilicon comprising the actuator arm or the metallic layer formed on theactuator arm, this metallic layer comprising the arm electrostaticplate. The unbalanced residual stresses can be generated by manipulationof deposition process parameters during formation of the actuator armstructure. Due to these residual stresses in the actuator arm structure,the actuator arm is in a stressed condition prior to release from thesacrificial layer and will tend to bend away from the substrate whenreleased. This counters the tendency of the arm to deflect toward thesubstrate in response to surface tension forces exerted by the releasesolution.

[0021] A general embodiment of the electromechanical switches accordingto the present invention has a cantilevered actuator arm which has anelectrostatic plate disposed above an electrostatic plate positioned ona substrate. The switch is open and closed by the electrostaticattraction between the plates. In the open position, the cantileveredarm curves away from the substrate. Switching is provided by a gappedtransmission line positioned on the substrate at one end of thecantilevered arm. The arm carries an electrical contact that bridges thegap when the switch is in the closed position. The electrical contactmay simply be a region of metal or other electrically conductingmaterial attached to the arm. The electrical contact may also compriseelectrically conducting material that projects through the arm tocontact the gapped transmission line when the switch is closed. Theelectrical contact may also be electrically isolated from the arm by alayer of insulating material disposed at the end of the arm. The arm mayalso be electrically isolated from the electrostatic plate on thesubstrate when the switch is closed by mechanical stops disposed next tothe electrostatic plate that prevent the arm from contacting the plate.

[0022] Embodiments of the switches according to the present inventionmay be fabricated by well-known integrated circuit fabricationprocesses. Generally, The processes also involve applying one or morelayers of sacrificial material. These layers of sacrificial materialsupport the fabrication of the desired structures for the switch. Otherprocesses involve applying one or more layers of electrically conductivematerial to form the electrically conductive elements, such as theelectrostatic plates and electrical contact. As briefly noted above, itis desired that the actuating arm of the cantilever structure accordingto the present invention be fabricated such that it curls or curvesupwards when the switch is open. Processes used to obtain this resultare described below.

[0023] An aspect of the present invention comprises: a substrate; afirst electrical contact formed on the substrate; a substrateelectrostatic plate formed on the substrate; a cantilever actuator armanchored to the substrate at a first end of the actuator arm; a secondelectrical contact disposed at a second end of the actuator arm, thesecond electrical contact being in electrically contact with the firstelectrical contact when the switch is in a closed position; thesubstrate electrostatic plate disposed beneath the actuator arm andbetween the first end and the second end of the actuator arm; and an armelectrostatic plate formed on the actuator arm and positioned above thesubstrate electrostatic plate when the switch is in a closed position,wherein when the switch is in an open position, the actuator arm curvesaway from the substrate.

[0024] Another embodiment of the present invention also provides amethod for switching electrical energy comprising providing anelectrostatically actuated cantilevered arm on a substrate; applying avoltage to attract the electrostatically actuated cantilevered armtowards the substrate, the cantilevered arm having an electrical contactthat electrically connects the input to the output when the voltage isapplied; and removing the voltage or applying a second voltage to causethe cantilevered arm to move away from the substrate, the electricalcontact no longer electrically connecting the input to the output whenthe voltage is removed or the second voltage is applied, such that thecantilevered arm curves away from the substrate when the voltage isremoved or the second voltage is applied.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a side view of the cantilever beam and contactarrangement of an RF MEMS switch according to a preferred embodiment ofthe present invention, showing the switch in the open position.

[0026]FIG. 2 is a side view similar to FIG. 1 showing the switch in theclosed position.

[0027]FIG. 3 is a plan view of the switch of FIG. 1 in the openposition.

[0028] FIGS. 4A-N are side views of the switch of FIG. 1 illustratingthe steps in fabricating the switch.

[0029]FIG. 5A is a side view of a second embodiment of an RF MEMS switchaccording to the present invention, showing the switch in the openposition.

[0030]FIG. 5B is a side view of the switch of FIG. 5A, showing theswitch in the closed position.

[0031]FIG. 6 is a plan view of the switch of FIGS. 5A and 5B, showingthe general size, shape, and orientation of the various layers of theswitch.

[0032] FIGS. 7A-F are side views of the switch of FIGS. 5A, 5B, and 6,illustrating the steps in fabricating the switch.

[0033]FIG. 8 is a side view of a third embodiment of an RF MEMS switchaccording to the present invention, showing the switch in the openposition.

[0034]FIG. 9 is a side view of the switch of FIG. 8, showing the switchin the closed position.

[0035] FIGS. 10A-10T are side views of the switch of FIGS. 8 and 9,illustrating the steps in fabricating the switch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] The present disclosure describes a miniature RF switch designedfor applications over a frequency range from DC to approximately 100GHz. The following disclosure describes an RF MEMS switch according tothe present invention fabricated on a silicon-based substrate. However,RF MEMS switches according to the present invention may also befabricated from various other substrate materials, such as galliumarsenide (GaAs), glass, and other dielectrics.

[0037] In a preferred embodiment, a micro-electromechanical switch,generally designated 124 and best illustrated in FIGS. 1, 2 and 3, isfabricated on a substrate 110 using generally known microfabricationtechniques, such as masking, etching, deposition, and lift-off. In apreferred embodiment, the RF MEMS switch 124 is directly formed on thesubstrate 110 and monolithically integrated with a transmission line114. Alternatively, the RF MEMS switch 124 may be discreetly formed andthen bonded to the substrate 110. The switch 124 comprises thetransmission line 114, a substrate electrostatic plate 120, an actuatingportion 126, and an electrical contact 134. The substrate electrostaticplate 120 (typically connected to ground) and the transmission line 114are formed on the substrate 110. An insulating layer 111 may be used toelectrically isolate the transmission line 114, the substrateelectrostatic plate 120, and the actuating portion 126 from thesubstrate 110. The substrate electrostatic plate 120 and thetransmission line 114 preferably comprise microstrips of a metal noteasily oxidized, e.g., gold, deposited or otherwise formed on thesubstrate 110. The transmission line 114 includes a gap 118 (shown inFIG. 3) that is opened and closed by operation of the switch 124, in amanner explained below.

[0038] The actuating portion 126 of the switch 124 comprises acantilever anchor 128 formed on the substrate 110, and a cantileveredactuator arm 130 extending from cantilever anchor 128. The actuator arm130 forms a suspended micro-beam projecting from one end at thecantilever anchor 128 and extending over and above the substrateelectrostatic plate 120 and the transmission line 114 on the substrate110.

[0039] The actuator arm 130 has a bilaminar cantilever (or bimorph)structure, that is, the structure comprises two dissimilar materials,preferably with different residual stresses, layered together. Due toits mechanical properties, the bimorph structure exhibits a very highratio of displacement to actuation voltage. That is, a relatively largedisplacement (approximately 300 micrometers) can be produced in thebimorph cantilever in response to a relatively low switching voltage(approximately 20 V). A first layer 136 of the actuator arm 130preferably comprises a semi-insulating or insulating material, such aspolycrystalline silicon. A second layer 132 of the actuating arm 130preferably comprises a metal film (typically aluminum or gold) depositedatop first layer 136.

[0040] As shown in FIG. 1, the second layer 132 comprises a firstportion 138 formed proximate the cantilever anchor 128 and a secondportion 140 extending from the first portion 138 toward the position onthe actuator arm 130 at which the electrical contact 134 is formed. Thesecond portion 140 typically acts as an arm electrostatic plate 140during operation of the switch 124. In the remainder of the description,the terms “second layer” and “arm electrostatic plate” will be usedinterchangeably. A third portion 139 of the second layer 132 may beformed within or on the cantilever anchor 128 to provide an electricalconnection to the second layer 132. At least a portion of the armelectrostatic plate 140 and a corresponding portion of the actuator arm130 on which the arm electrostatic plate 140 is formed are positionedabove the substrate electrostatic plate 120 to form an electrostaticallyactuatable structure. The height of the cantilever anchor 128 and thefirst layer 136 above the substrate 110 can be tightly controlled usingknown fabrication methods. Forming the arm electrostatic plate 140 ontop of the first layer 136 allows a correspondingly high degree ofcontrol over the height of the arm electrostatic plate 140 above thesubstrate electrostatic plate 120. As the switch actuation voltage isdependent upon the distance between the substrate electrostatic plate120 and the arm electrostatic plate 140, a high degree of control overthe spacing between the electrostatic plates 120, 140 is preferred inorder to repeatably achieve a desired actuation voltage.

[0041] The electrical contact 134, typically comprising a metal thatdoes not easily oxidize, e.g., gold, platinum, or gold palladium, isformed on the actuator arm 130 and positioned on the arm 130 so as toface the gap 118 formed in the transmission line 114. When the switch124 is in the closed position, the electrical contact 134 bridges thegap 118 and provides an electrical connection between the two portionsof the transmission line 114.

[0042] To achieve a low actuation voltage without sacrificing electricalisolation in the OFF-state (or open switch state), the actuator arm 130is formed so it bends or curls upwards and away from the substrate 110.Preferably, the upwards curl in the actuator arm 130 is such that theminimum separation distance between the transmission line 114 and theelectrical contact 134 formed on the actuator arm 130 is equal to orgreater than the maximum separation distance between the substrateelectrostatic plate 120 and the arm electrostatic plate 140 when theswitch 124 is in the open position. The upwards curl in the actuator arm130 is caused by nonuniform residual stresses induced in the materialcomprising the first layer 136 of the actuator arm 130, the second layer132, or both layers 132, 136 during the fabrication of those layers.Alternatively, the curve in the actuator arm may be induced by usingmaterials with different residual stresses for the first layer 136 andthe second layer 132 of the actuator arm 130. The different residualstresses may result from different properties of the first layer 136 andthe second layer 132, such as different coefficients of thermalexpansion.

[0043] The operation of the preferred embodiment will now be discussedwith reference to FIGS. 1-3. In operation, the switch 124 is normally inan open or “OFF” position as shown in FIG. 1. With the switch 124 in theOFF-state, the transmission line 114 is an open circuit due to both thegap 118 and the separation of the electrical contact 134 from thetransmission line 114.

[0044] The switch 124 is actuated to the closed or “ON” position byapplication of a voltage between the arm electrostatic plate 140 and thesubstrate electrostatic plate 120. When the voltage is applied, the armelectrostatic plate 140 is electrostatically attracted toward thesubstrate electrostatic plate 120, forcing the actuator arm 130 todeflect toward the substrate 110. Deflection of the actuator arm 130toward the substrate electrostatic plate 120, as indicated bydouble-headed arrow 11 in FIG. 1, causes the electrical contact 134 tocome into contact with the transmission line 114, thereby bridging thegap 118 and placing the transmission line 114 in an ON-state (i.e.,closing the circuit). As previously explained, the arm electrostaticplate 140 is formed at a point on the actuator arm 130, (for example,adjacent the cantilever anchor 128) which allows the distance betweenthe arm electrostatic plate 140 and the substrate electrostatic plate120 formed on the substrate 110 to be precisely and repeatablycontrolled using standard photolithographic processes.

[0045] Furthermore, in the “OFF” or open state, the actuator arm 130curls upwards so that the minimum separation distance between thetransmission line 114 and the electrical contact 134 formed on theactuator arm 130 is preferably equal to or greater than the maximumseparation distance between the substrate electrostatic plate 120 andthe arm electrostatic plate 140. Thus, the distance between theelectrical contact 134 and the transmission line 114 formed on thesubstrate 110 is greater than the corresponding spacing characteristicof conventional MEMS cantilever-type switches (such as those disclosedby Yao and Loo et al., as discussed previously). As a result, theOFF-state capacitance of the switch is greatly reduced.

[0046] The actuation voltage required to close the switch 124 isprimarily determined by the distance between the substrate electrostaticplate 120 and the portion of the arm electrostatic plate 140 disposedclosest to the substrate electrostatic plate 120. Since the distanceseparating the arm electrostatic plate 140 and the substrateelectrostatic plate 120 formed on the substrate 110 is precisely andrepeatably controlled, the voltage required to cause the actuator arm130 to snap down can be correspondingly controlled and minimized tomaintain a relatively low actuation voltage. Further, due to the curl ofthe actuator arm 130, a zipper-like actuation motion is produced uponapplication of the actuation voltage. That is, the end of the armelectrostatic plate 140 closest to the cantilever anchor 128 willinitially be attracted towards the substrate electrostatic plate 120.The motion of this end towards the substrate electrostatic plate 120will decrease the distance of the remainder of the arm electrostaticplate 140 from the substrate electrostatic plate 120, which furtherdecreases the voltage required to close the switch 124. Therefore, theoverall motion of the actuator arm 130 as it moves towards the substrateelectrostatic plate 120 appears much like the motion of a zipper as itis closed.

[0047] Embodiments of the present invention provide the importantadvantage of reduced OFF-state capacitance without a correspondingincrease in actuation voltage. Thus, the actuation voltage and the RFperformance of the switches according to the present invention can beseparately optimized.

[0048] One possible method of fabricating the switch 124 will now bedescribed. The switch 124 may be manufactured using generally knownmicrofabrication techniques, such as masking, etching, deposition, andlift-off. For example, the switch 124 may be fabricated using afoundry-based polysilicon surface-micromachining process, or ametal/insulator surface-micromachining process. The substrate 110 forone preferred embodiment may be a semi-insulating GaAs wafer, althoughother materials such as InP, ceramics, quartz or silicon may be used.Polycrystalline silicon deposited using plasma enhanced chemical vapordeposition may be used as the preferred structural material forcantilever anchor 128 and actuator arm 130, and silicon dioxide may beused as sacrificial material, as described below.

[0049] FIGS. 4A-N are side view schematic illustrations of a processsequence that may be used to fabricate the switch 124 illustrated inFIGS. 1-3. Note that the insulating layer 111 is not shown in FIGS.4A-N, but alternative fabrication processes may include this feature.Note also that the switch 124 may be fabricated by processes other thanthose depicted in FIGS. 4A-N. Further, while FIGS. 4A-N depict multipleseparate fabrication steps, alternative fabrication processes may allowseveral separate steps to be combined into fewer steps. Finally,alternative fabrication processes may use a different sequence of steps.

[0050] The fabrication of the switch 124 may begin with the fabricationof the substrate electrostatic plate 120. Prior to forming the substrateelectrostatic plate 120, a first layer 142 of sacrificial material, suchas silicon dioxide, is formed on the substrate 110, as shown in FIG. 4A.A hole 121 is then etched in the first sacrificial layer 142 toaccommodate the substrate electrostatic plate 120, as shown in FIG. 4B.A first layer of gold (or other conductor) is preferably deposited usingelectron beam evaporation and liftoff to form the substrateelectrostatic plate 120, as shown in FIG. 4C.

[0051] The gapped transmission line 114 may then be formed. A secondlayer 146 of sacrificial material, such as silicon dioxide, isdeposited, as shown in FIG. 4D. A hole 115 at one side of the switch 124is then etched through the first sacrificial layer 142 and the secondsacrificial layer 146 for a first portion of the transmission line 114,as shown in FIG. 4E. A corresponding hole (not shown in FIGS. 4A-4N) onthe other side of the switch 124 is also etched through the firstsacrificial layer 142 and the second sacrificial layer 146 for a secondportion of the transmission line 114. The first and second portions ofthe transmission line 114 are separated by the gap 118. A second layerof conductive material, such as gold, is preferably deposited usingelectron beam evaporation and liftoff to form the first and secondportions of the transmission line 114, as shown in FIG. 4F.

[0052] The electrical contact 134 may then be formed. A thirdsacrificial layer 148 is deposited on top of the second sacrificiallayer 146 and portions of the transmission line 114, as shown in FIG.4G. A hole 135 is then preferably partially etched in the thirdsacrificial layer 148, as shown in FIG. 4H. A third layer of conductivematerial, such as gold, is preferably deposited using electron beamevaporation and liftoff to form the electrical contact 134, as shown inFIG. 41.

[0053] The actuating portion 126 may then be formed. A hole 137 isetched through the first sacrificial layer 142, the second sacrificiallayer 146, and the third sacrificial layer 148 to form the location forthe cantilever anchor 128, as shown in FIG. 4J. A layer ofpolycrystalline silicon 136 is then deposited atop the sacrificiallayers 142, 146, 148 to form the cantilever anchor 128 and the actuatorarm 130, as shown in FIG. 4K. Preferably, the residual stresses in thepolycrystalline silicon are used to control the extent of the upwardscurl in the actuator arm 130 when the switch 124 is in the OFF-state.Process factors affecting the residual stresses in polycrystallinesilicon during the deposition and release phases include the structureof the deposited layer (i.e., the degree of crystallinity), the textureof the layer, the thickness of the layer, the speed at which the layerdeposition process occurs and the presence or absence of doping. Also,during the release phase, residual stresses in polycrystalline siliconare affected by time of exposure to release agents. By controlling thesefactors, the residual stresses in the polycrystalline silicon layer 136may be affected.

[0054] The arm electrostatic plate 140 may then be formed. The secondlayer 132, comprising a metal, for example, aluminum, is deposited usingelectron beam evaporation and liftoff to form the arm electrostaticplate 140 on the actuator arm 130, as shown in FIG. 4L. As previouslydescribed, the arm electrostatic plate 140 is formed so that it issubstantially above the substrate electrostatic plate 120 to maximizethe electrostatic attraction between the two plates 120, 140.

[0055] Fabrication of the switch is completed by using chemical releasemethods known in the art to remove the sacrificial layers 142, 146, 148.FIG. 4M shows the switch 124 after the sacrificial layers 142, 146, 148have been removed, but without the desired curl in the actuator arm 130.Removal of the sacrificial layers 142, 146, 148 should actually resultin the actuator arm 130 curling upwards, as shown in FIG. 4N.

[0056]FIGS. 5A and 5B depict side views of an alternate embodiment of anRF MEMS switch 224 according to the present invention. In thisembodiment, a cantilever actuator arm 230 comprises both a first armstructural layer 236 constructed of an insulating material and a secondarm electrostatic plate layer 232 constructed of a conducting material.FIG. 5A depicts the RF MEMS switch 224 in the open or OFF-state. FIG. 5Bdepicts the RF MEMS switch 224 in the closed or ON-state. FIG. 6 is atop plan view of this embodiment that shows the orientation of thevarious elements of this embodiment, discussed in additional detailbelow.

[0057] The RF MEMS switch 224 is fabricated upon a substrate 210,preferably GaAs, although other materials may be used, such as InP,ceramics, quartz or silicon. The material for the substrate 210 ischosen primarily based on the technology of the circuitry the RF MEMSswitch 224 is to be connected to so that the switch 224 and thecircuitry may be fabricated simultaneously. For example, InP can be usedfor low noise HEMT MMICS (high electron mobility transistor monolithicmicrowave integrated circuits) and GaAs is typically used for PHEMT(pseudomorphic HEMT) power MMICS.

[0058] The switch 224 comprises a transmission line 214, a substrateelectrostatic layer 220, and a cantilever actuator arm 230. Thecantilever actuator arm 230 comprises the arm structural layer 236, thearm electrostatic plate layer 232, and a conducting transmission line234 with at least one dimple 235 that preferably protrudes below the armstructural layer 236. FIG. 6 shows that the cantilever actuator 230 mayhave two dimples 235, while alternative embodiments may have more thantwo dimples 235 or a single dimple that is disposed beneath the lengthof the conducting transmission line 234. The arm electrostatic platelayer 232 connects to an arm plate layer contact 228 at the base of thecantilever actuator arm 230 to provide an electrical connection betweenthe arm electrostatic plate layer 232 and the arm plate layer contact228. The substrate electrostatic layer 220 contains a substrateelectrostatic plate 222 located generally beneath the cantileveractuator arm 230. The arm electrostatic layer 232 contains an armelectrostatic plate 238 located generally above the substrateelectrostatic plate 222. An application of a voltage between the armelectrostatic plate 238 and the substrate electrostatic plate 222 willcause the plates to be electrostatically attracted. When the plates 222,238 are electrostatically attracted together, the switch 224 is in theclosed position and the dimples 235 are in electrical contact with thetransmission line 214. Since the dimples 235 are electrically connectedtogether by the conducting transmission line 234, the electrical contactof the dimples 235 with the transmission line 214 bridges the gap 218 inthe transmission line 214 when the switch 224 is in the closed position.

[0059] One possible method of fabricating switch 224 is discussed below.As previously discussed, an advantage of the present invention is thatit can be manufactured using standard integrated circuit fabricationtechniques. The switch 224 can also be fabricated on wafers that containother integrated circuit devices. The flexibility in the fabrication ofthis and other embodiments of the present invention allows the presentinvention to be used in a variety of circuits. Note also that the sameor similar materials for the layers and thicknesses for the layersdiscussed below may also be used in the fabrication of the switch 124discussed above, along with the same or similar fabrication steps.

[0060]FIG. 7A shows a profile of the MEMS switch 224 after the firststep of depositing a first metal layer onto the substrate 210 for thearm plate layer contact 228, the transmission line 214, and thesubstrate electrostatic layer 220 is complete. The first metal layer maybe deposited lithographically using standard integrated circuitfabrication technology, such as resist lift-off or resist definition andmetal etch. In the preferred embodiment, gold (Au) is used as theprimary composition of the first metal layer. Au is preferred in RFapplications because of its low resistivity. In order to ensure theadhesion of the Au to the substrate, a thin layer (preferably about250-500 angstroms) of titanium (Ti) is deposited, followed by preferablyabout a 1000 angstrom layer of platinum (Pt), and finally the Au. The Ptacts as a diffusion barrier to keep the Au from intermixing with the Tiand causing the metal to lose adhesion strength to the substrate 210. Inthe case of a group III-V semiconductor substrate, a thin layer of goldgermanium (AuGe) eutectic metal may be deposited first to ensureadhesion of the Au by alloying the AuGe into the semiconductor, similarto a standard ohmic metal process for any group III-V MESFET or HEMT.

[0061] Next, as shown in FIG. 7B, a sacrificial layer 242 is placed ontop of the first metal layer and etched so that the cantilever actuatorarm 230 may be produced above the sacrificial layer 242. The sacrificiallayer 242 is typically comprised of 2 microns of SiO₂ which may besputter deposited or deposited using PECVD (plasma enhanced chemicalvapor deposition). A via 243 is etched in the sacrificial layer 242 sothat the metal of the arm plate layer contact 228 is exposed. The via243 definition may be performed using standard resist lithography andetching of the sacrificial layer 242. Other materials besides SiO₂ maybe used as a sacrificial layer 242. The important characteristics of thesacrificial layer 242 are a high etch rate, good thickness uniformity,and conformal coating by the layer 242 of the metal already on thesubstrate 210. The thickness of the layer 242 partially determines theinitial thickness of the switch opening, before the arm 230 begins tocurve away from the substrate 210 due to residual stresses. Thesacrificial layer 242 will be removed in the final step to release thecantilever actuator arm 230, as shown in FIG. 7F.

[0062] Another advantage of using SiO₂ as the sacrificial layer 242 isthat SiO₂ can withstand high temperatures. Other types of supportlayers, such as organic polyimides, harden considerably if exposed tohigh temperatures. This makes a polyimide sacrificial layer difficult tolater remove. The sacrificial layer 242 is exposed to high temperatureswhen the preferred material of silicon nitride for the arm structurallayer 236 is deposited (as shown in FIG. 7C), as a high temperaturedeposition is desired when depositing the silicon nitride to give thesilicon nitride a lower buffered oxide etch (BOE) etch rate. A low BOEetch rate minimizes the amount of the arm structural layer 236 that islost when the SiO₂ is etched away.

[0063]FIG. 7C shows the fabrication of the arm structural layer 236. Thearm structural layer 236 is the supporting mechanism of the cantileveractuator arm 230 and is preferably made out of silicon nitride, althoughother materials besides silicon nitride may be used. The material usedfor the arm structural layer 236 should have a low etch rate compared tothe sacrificial layer 242 so that the arm structural layer 236 is notetched away when the sacrificial layer 242 is removed to release thecantilever actuator arm 230. The arm structural layer 236 is patternedand etched using standard lithographic and etching processes.

[0064] The arm structural layer 236 is preferably formed below the armelectrostatic plate layer 232. Since the arm structural layer 230 isfabricated on only one side of the arm electrostatic plate layer 232,bowing will occur in the cantilever actuator arm 230 when the arm 230 isreleased if the residual stress in the arm structural layer 236 differsfrom the stress in the arm electrostatic plate layer 232. The materialsin the arm structural layer 236 and the arm electrostatic plate layer232 are chosen such that the differing stresses in the materials causethe arm 230 to bow upwards. The techniques used to deposit the armelectrostatic plate layer 232 also affect the amount of curvatureachieved.

[0065] In FIG. 7D, a dimple receptacle 253 is etched into the armstructural layer 236 and the sacrificial layer 242. The dimplereceptacle 253 is an opening where the dimple 235 will later bedeposited. The dimple receptacle 253 is created using standardlithography and a dry etch of the arm structural layer 236, followed bya partial etch of the sacrificial layer 242. The opening in thesacrificial layer 242 allows the dimple 235 to preferably protrudethrough the sacrificial layer 242. Note that a plurality of dimplereceptacles 253 may be formed to allow a plurality of dimples 235 to beused to form an electrical contact with the transmission line 214 whenthe switch 224 is in the closed position.

[0066] Next, a second metal layer is deposited onto the arm structurallayer 236. The second metal layer forms the arm electrostatic platelayer 232, the conducting transmission line 234, and the dimple 235. Ina preferred embodiment, the second metal layer is comprised of a sputterdeposition of a thin film, preferably about 200 angstroms, of Ti,preferably followed by about a 1000 angstrom deposition of Au. Thesecond metal layer must be conformal across the wafer and acts as aplating plane for the Au. The plating is done by using metal lithographyto open up the areas of the switch that are to be plated. The Au may beelectroplated by electrically contacting the membrane metal on the edgeof the wafer and placing the metal patterned wafer in the platingsolution. The plating occurs only where the membrane metal is exposed tothe plating solution to complete the electrical circuit and not wherethe electrically insulating resist is left on the wafer. After about 2microns of Au is plated, the resist is stripped off of the wafer and thewhole surface is ion milled to remove the membrane metal. Some Au willalso be removed from the top of the plated Au during the ion milling,but that loss is minimal because the membrane is preferably only 1200angstroms thick.

[0067] As shown in FIG. 7E, the result of this process is that theconducting transmission line 234 and the dimple 235 are created by thesecond metal layer, which comprises Au in a preferred embodiment. Inaddition, the Au fills the via 251 and connects the arm electrostaticplate layer 232 to the arm plate layer contact 228. Au is a preferredchoice for the second metal layer because of its low resistivity. Whenchoosing the metal for the arm electrostatic layer 232 and the materialfor the arm structural layer 236, it is important to select thematerials such that the stress of the arm structural layer 236 variesfrom the stress of the arm electrostatic layer 232 so that thecantilever actuator arm 230 will bow upwards when the switch 224 is inthe OFF-state. This is done by carefully determining the depositionparameters for the structural layer 236. Silicon nitride was chosen forthis structural layer 236 not only for its insulating properties, but,in large part, because of the controllability of these depositionparameters and the resultant stress levels of the layer.

[0068] The arm structural layer 236 is then lithographically defined andetched to complete the switch fabrication. Finally, the sacrificiallayer 242 is removed to release the cantilever actuator arm 230. If thesacrificial layer 242 is comprised of SiO₂, then it will typically bewet etched away in the final fabrication sequence by using ahydrofluoric acid (HF) solution. The etch and rinses are performed withpost-processing in a critical point dryer to ensure that the cantileveractuator arm 230 does not come into contact with the substrate 210 whenthe sacrificial layer 242 is removed. If contact occurs during thisprocess, device sticking and switch failure are probable. Note, however,that the bimorph character of the cantilever actuator arm 230 shouldcause the arm 230 to bow upwards and should also reduce the likelihoodof contact with the substrate 210 upon removal of the sacrificial layer242. Contact may be prevented by transferring the switch from a liquidphase (e.g. HF) environment to a gaseous phase (e.g. air) environmentnot directly, but by introducing a supercritical phase in between theliquid and gaseous phases. The sample is etched in HF and rinsed withde-ionized (DI) water by dilution, so that the switch is not removedfrom a liquid during the process. DI water is then replaced withmethanol. The sample is transferred to the critical point dryer and thechamber is sealed. High pressure liquid CO₂ replaces the methanol in thechamber, so that there is only CO₂ surrounding the sample. The chamberis heated so that the CO₂ changes into the supercritical phase. Pressureis then released so that the CO₂ changes into the gaseous phase. Nowthat the sample is surrounded only by gas, it may be removed from thechamber into room air. A side elevational view of the MEMS switch 224after the support layer 242 has been removed, but before the cantileveractuator arm 230 curls upwards, is shown in FIG. 7F.

[0069]FIGS. 8 and 9 depict views of another embodiment of an RF MEMSswitch 324 according to the present invention. In this embodiment, aninsulating layer 340 electrically isolates a conducting transmissionline 334 from an arm structural layer 336 and an arm electrostatic plate332. FIG. 8 depicts the RF MEMS switch 324 in the open or OFF-state.FIG. 9 depicts the RF MEMS switch 324 in the closed or ON-state.

[0070] The switch 324 comprises a transmission line 314, a substrateelectrostatic plate 320, and a cantilever 326. The switch 324 isfabricated upon a substrate 310, preferably comprising GaAs, althoughother materials may be used, such as InP, ceramics, quartz or silicon.The substrate 310 may also be coated with an insulating layer 311comprising, for example, silicon nitride. The cantilever 326 comprises acantilever anchor 328 fabricated on the substrate 310 and a cantileveractuator arm 330. The cantilever actuator arm 330 comprises the armstructural layer 336, the arm electrostatic plate 332, an insulatinglayer 340, and the conducting transmission line 334. Preferably, thecantilever anchor 328 also is integral with the arm structural layer336.

[0071] The substrate electrostatic plate 320 is formed on the substrate310 and is located generally beneath the arm electrostatic plate 332 onthe cantilever actuator arm 330. An application of a voltage between thearm electrostatic plate 332 and the substrate electrostatic plate 320will cause the plates 320, 332 to be electrostatically attracted.Preferably, mechanical stops 316 are formed on the substrate that areelectrically isolated from the substrate electrostatic plate 320 andhave a greater height than the substrate electrostatic plate 320. Themechanical stops 316 prevent the cantilever actuator arm 330 from cominginto electrical contact with the substrate electrostatic plate 320. Aninsulating, semi-conducting, or conducting layer 313 may be locatedbeneath the transmission line 314 to isolate the transmission line fromthe substrate 310 or to decrease the amount of deflection required forthe conducting transmission line 334 to contact the transmission line314.

[0072] One possible method of fabricating switch 324 is discussed below.This embodiment of the present invention is particularly adapted forfabrication by using standard three-polysilicon-layersurface-micromachining processes, such as that provided by theMulti-User MEMS Processes (MUMPs™) from Cronos Integrated Microsystemsof Research Triangle Park, N.C. However, other methods of micromachiningfabrication may be used.

[0073]FIG. 10A shows a cross-section of an embodiment of the switch 324at the beginning of the fabrication process. The surface of the startingn-type silicon substrate 310 is heavily doped with phosphorus in astandard diffusion furnace using POCl₃ as the dopant source. Preferably,a blanket layer 311, about 0.5 μm thick, of low stress silicon nitrideis deposited on the substrate 310 as an insulating layer. Then,preferably, a polysilicon layer 320 (POLY0), about 0.5 μm-thick, isdeposited for providing the conducting surfaces for the substrateelectrostatic plate 322 and the conducting layer 313 for the gappedtransmission line 314. The wafer is then coated with anultraviolet-sensitive photoresist layer 390.

[0074] The photoresist layer 390 is lithographically patterned byexposing it to ultraviolet light through a first level mask and thendeveloping it. The photoresist layer 390 in exposed areas is removedleaving behind a patterned photoresist mask for etching, as shown inFIG. 10B. Reactive Ion Etching (RIE) is used to remove any unwantedpolysilicon. After the etch, the photoresist is chemically stripped in asolvent bath. This method of patterning the wafers with photoresist,etching and stripping the remaining photoresist is also used to removeunwanted portions of the additional polysilicon layers described below.

[0075] After the unwanted polysilicon is removed from the POLY0 layer320 and the photoresist is removed, the conducting surfaces for thesubstrate electrostatic plate 322 and the conducting layer 313 remain,as shown in FIG. 10C. A blanket layer 371, approximately 2.0 μm thick,of phosphosilicate glass (PSG) is deposited on the structure by lowpressure chemical vapor deposition (LPCVD). The deposit of this firstsacrificial layer 371 is shown in FIG. 10D. Other materials such as SiO₂may also be used for the sacrificial layer 371.

[0076] The structure is then coated with photoresist and the areas 381for the mechanical stops 316 are lithographically patterned. Theseareas, which reach the nitride layer 311, are reactive ion etched intothe first sacrificial layer 371. After the etch, the photoresist isstripped. The structure after removal of the photoresist is shown inFIG. 10E.

[0077] A second layer 317 (POLY1), approximately 2.0 μm thick, ofun-doped polysilicon is deposited by LPCVD. This layer serves to fill inthe mechanical stop areas 381. The results of this step are shown inFIG. 10F. The POLY1 layer 317 is again coated with photoresist,patterned, and etched. The result is to remove the bulk of the POLY1layer 317 on top of the sacrificial layer 371, as shown in FIG. 10G. Asecond sacrificial layer 372, preferably comprising PSG, is thendeposited on top of the structure, as shown in FIG. 10H.

[0078] The structure is again coated with photoresist and the anchorarea 329 for the cantilever anchor 328 is etched into both the secondsacrificial layer 372 and first sacrificial layer 371 down to thenitride layer 311, as shown in FIG. 10I. A third polysilicon layer 336(POLY2) is then deposited onto the structure by LPCVD, as shown in FIG.10J. The POLY2 layer 336 provides the cantilever arm structural layer336 and the cantilever anchor 328.

[0079] The POLY2 layer 336 is then etched to provide access to the area341 to be used for the gapped transmission line 314, as shown in FIG.10K. An additional mask and etch process is used to remove portions ofthe first sacrificial layer 371 and the second sacrificial layer 372 toexpose the conducting layer 313, as shown in FIG. 10L. The structure isthen coated with photoresist and a metal layer is lithographicallypatterned for the arm electrostatic plate 332 and the gappedtransmission line 314. The metal layer, preferably gold with a thinadhesion layer, is deposited by lift-off patterning. The photoresist andany unwanted metal are then removed in a solvent bath. The structureafter removal of unwanted metal is shown in FIG. 10M.

[0080] A third sacrificial layer 373, also preferably comprising PSG, isthen provided over portions of the arm electrostatic plate 332, the armstructural layer 336, the gapped transmission line 314, the firstsacrificial layer 371 and the second sacrificial layer 372, as shown inFIG. 10N. The third sacrificial layer 373 is then etched to provide anarea 335 for the conducting transmission line 334. Metal, againpreferably gold with a thin adhesion layer, is deposited to create theconducting transmission line 334 and then any unwanted portions of metalare removed. The structure after creation of the conducting transmissionline is shown in FIG. 100.

[0081] Additional portions of the third sacrificial layer 373 are thenremoved to provide for mechanically coupling the conducting transmissionline 334 to the arm structural layer 336. The structure after removal ofportions of the third sacrificial layer is shown in FIG. 10P. Aninsulating material layer 340, such as silicon nitride, is deposited ontop of the conducting transmission line 334 and proximate to the armstructural layer 336. The insulating material layer 340 essentiallycauses the conducting transmission line 334 to be fixedly attached tothe arm structural layer 336, as shown in FIG. 10Q. Portions of theinsulating material layer 340 may be removed as shown in FIG. 10R. Theremoval of portions of the insulating material layer 340 may be done todecrease the weight of the cantilever actuator arm 330 and to allow thecantilever actuator arm 330 to curl upwards as desired.

[0082] Finally, the cantilever actuator arm 330 is released by removingthe sacrificial layers 371, 372, 373. The release may be performed byimmersing the structure in a bath of 49% hydrofluoric acid at roomtemperature for 1.5 to 2 minutes. Other methods known in the art mayalso be used to remove the sacrificial layers 371, 372, 373. This isfollowed by several minutes in DI water and then alcohol to reduce thelikelihood of stiction. Finally, the structure is placed in an oven at1100° C. for at least 10 minutes. The structure after removal of thesacrificial material is shown in FIG. 10S. Note that due to the bimorphcharacter of the switch, the cantilever actuator arm 330 should curlupwards as shown in FIG. 10T.

[0083] Although the present invention has been described with respect tospecific embodiments thereof, various changes and modifications can becarried out by those skilled in the art without departing from the scopeof the invention. In particular, the substrate, cantilever anchor,cantilever arm, electrostatic plates, and metal contacts may befabricated using any of various materials appropriate for a given enduse design. The cantilever anchor, cantilever arm, electrostatic plates,and metal contacts may be formed in various configurations, includingmultiple anchor points, cantilever arms, and metal contacts. It isintended, therefore, that the present invention encompass such changesand modifications as fall within the scope of the appended claims.

What is claimed is:
 1. An electromechanical switch comprising: asubstrate; a first electrical contact formed on said substrate; asubstrate electrostatic plate formed on said substrate; a cantileveractuator arm anchored to said substrate at a first end of said actuatorarm; a second electrical contact disposed at a second end of saidactuator arm, said second electrical contact being in electricallycontact with said first electrical contact when said switch is in aclosed position; said substrate electrostatic plate disposed beneathsaid actuator arm and between said first end and said second end of saidactuator arm; and an arm electrostatic plate formed on said actuator armand positioned above said substrate electrostatic plate when said switchis in a closed position, wherein when said switch is in an openposition, said actuator arm curves away from said substrate.
 2. Theswitch of claim 1, wherein said first electrical contact has a gap thatis closed by the second electrical contact when the switch is in aclosed position.
 3. The switch of claim 1, wherein said actuator armcomprises polycrystalline silicon.
 4. The switch of claim 1, whereinsaid actuator arm is fabricated such that said actuator arm curves awayfrom said substrate due to nonuniform stresses in said actuator arm. 5.The switch of claim 1, wherein one or more mechanical stops are formedon said substrate, said one or more mechanical stops preventing saidactuator arm from contacting said substrate electrostatic plate whensaid switch is in the closed position.
 6. A method of switchingelectrical energy between an input and an output, the method comprisingthe steps of: providing an electrostatically actuated cantilevered armon a substrate; applying a voltage to attract said electrostaticallyactuated cantilevered arm towards said substrate, said cantilevered armhaving an electrical contact that electrically connects said input tosaid output when the voltage is applied; and removing said voltage orapplying a second voltage to cause said cantilevered arm to move awayfrom said substrate, said electrical contact no longer electricallyconnecting said input to said output when the voltage is removed or thesecond voltage is applied, wherein said cantilevered arm curves awayfrom said substrate when the voltage is removed or the second voltage isapplied.
 7. The method of claim 6, wherein said cantilevered arm curvesaway from said substrate due to nonuniform stresses in said cantileveredarm.
 8. The method of claim 6, wherein a substrate electrostatic plateis disposed on said substrate and an arm electrostatic plate is disposedon said cantilevered arm, said arm electrostatic plate positioned abovesaid substrate electrostatic plate when said cantilevered arm isattracted towards said substrate.
 9. The method of claim 8, wherein oneor more mechanical stops are disposed proximate to said substrateelectrostatic plate, said one or more mechanical stops preventing thecantilevered arm from contacting the substrate electrostatic plate whenthe cantilevered arm is attracted towards said substrate. 10 The methodof claim 6, wherein said electrical contact comprises two or moredimples projecting through said cantilevered arm, said two or moredimples being electrically connected together.
 11. Amicro-electromechanical switch formed on a substrate, said switchcomprising: a transmission line formed on said substrate, saidtransmission line having a transmission line gap forming an opencircuit; a substrate electrostatic plate formed on said substrate; andan actuating portion, said actuating portion comprising: a cantileveranchor formed on said substrate; a cantilevered actuator arm extendingfrom said cantilever anchor; an electrical contact formed on saidactuator arm and positioned facing said gap in said transmission line;and an arm electrostatic plate formed on said actuator arm, said armelectrostatic plate having a first portion formed proximate saidcantilever anchor and a second portion extending from said first portionalong said actuator arm, wherein when said switch is in an openposition, said actuator arm has a bend such that a minimum separationdistance between said transmission line and said electrical contact isequal to or greater than a maximum separation distance between saidsubstrate electrostatic plate and said arm electrostatic plate, said armelectrostatic plate and a segment of said actuator arm on which said armelectrostatic plate is formed defining a structure electrostaticallyattractable toward said substrate electrostatic plate upon selectiveapplication of a voltage to said arm electrostatic plate.
 12. Themicro-electromechanical switch of claim 11, wherein the electrostaticattraction of said electrostatically attractable structure toward saidsubstrate electrostatic plate causes said electrical contact on saidactuator arm to bridge said transmission line gap.
 13. Themicro-electromechanical switch of claim 11, wherein said substratecomprises a semi-insulating substrate.
 14. The micro-electromechanicalswitch of claim 13, wherein said semi-insulating substrate comprisesgallium arsenide (GaAs).
 15. The micro-electromechanical switch of claim11, wherein said actuator arm comprises polycrystalline silicon.
 16. Themicro-electromechanical switch of claim 11 wherein said bend in saidactuator arm is produced by inducing a nonuniform level of residualstress in said arm electrostatic plate formed on said actuator arm. 17.The micro-electromechanical switch of claim 11 wherein said bend in saidactuator arm is produced by inducing a nonuniform level of residualstress in said actuator arm.
 18. The micro-electromechanical switch ofclaim 11, wherein said substrate electrostatic plate and saidtransmission line comprise gold microstrips on said substrate.
 19. Themicro-electromechanical switch of claim 11, wherein said electricalcontact comprises a metal selected from the group consisting of gold,platinum, and gold palladium.
 20. A method for minimizing capacitivecoupling between elements of a micro-electromechanical switch formed ona substrate, said method comprising: forming a substrate electrostaticplate on said substrate; forming a transmission line on said substrate,said transmission line having a gap forming an open circuit; forming anactuating portion, said actuating portion comprising: a cantileveranchor formed on said substrate; and a cantilevered actuator armextending from said cantilever anchor above said substrate, saidactuator arm having an arm electrostatic plate formed thereon, saidactuator arm having an electrical contact facing said transmission line,wherein when said switch is in an open position said actuator arm has abend such that a minimum separation distance between said transmissionline and said electrical contact is greater than or equal to a maximumseparation distance between said substrate electrostatic plate and saidarm electrostatic plate, whereby capacitive coupling between saidtransmission line and said electrical contact is minimized by maximizingthe distance between said transmission line and said electrical contactformed on said actuator arm.
 21. The method of claim 20, wherein saidsubstrate comprises a semi-insulating substrate.
 22. The method of claim21, wherein said semi-insulating substrate comprises gallium arsenide(GaAs).
 23. The method of claim 20, wherein said actuator arm comprisespolycrystalline silicon.
 24. The method of claim 20 wherein said bend insaid actuator arm is produced by inducing a nonuniform level of residualstress in said arm electrostatic plate formed on said actuator arm. 25.The method of claim 20 wherein said bend in said actuator arm isproduced by inducing a nonuniform level of residual stress in saidactuator arm.
 26. The method of claim 20, wherein said substrateelectrostatic plate and said transmission line comprise gold microstripson said substrate.
 27. The method of claim 20, wherein said electricalcontact comprises a metal selected from the group consisting of gold,platinum, and gold palladium.
 28. A method of fabricating anelectromechanical switch on a substrate comprising the steps of: a)applying electrically conductive material to said substrate to form asubstrate electrostatic plate region, an input transmission line region,and an output transmission line region, said regions being electricallyisolated from one another; b) depositing one or more sacrificial layersover said regions; c) forming an electrical contact region on said oneor more sacrificial layers, said electrical contact region being appliedabove said input transmission line region and said output transmissionline region and said electrical contact region comprising electricallyconductive material; d) forming an actuating arm on said one or moresacrificial layers, said actuating arm contacting said substrate at afirst end and contacting said second layer of electrically conductivematerial at a second end; e) forming an arm electrostatic plate regionon said actuating arm, said arm electrostatic plate region beingdisposed above said substrate electrostatic pate region and comprisingelectrically conductive material; and f) removing said one or moresacrificial layers, wherein said steps of forming an actuating armand/or forming an arm electrostatic plate region are performed to inducenonuniform stress in said actuating arm so that actuating arm will bowupwards when the one or more sacrificial layers are removed.
 29. Themethod of claim 28, wherein the one or more sacrificial layers comprisesilicon dioxide.
 30. The method of claim 28, wherein the actuating armcomprises polycrystalline silicon.
 31. The method of claim 28, whereinsaid arm electrostatic plate region comprises aluminum.
 32. The methodof claim 28, wherein said electrical contact region, said inputtransmission line region, and said output transmission line regioncomprise gold.
 33. A micro-electromechanical switch formed on asubstrate comprising: a substrate transmission line formed on saidsubstrate, said substrate transmission line having a gap forming an opencircuit; a substrate electrostatic plate formed on said substrate; andan actuating portion, said actuating portion comprising: a cantileveranchor formed on said substrate; a cantilevered actuator arm extendingfrom said cantilever anchor, said actuator arm having an upper side anda lower side; a conducting transmission line formed on said upper sideof said actuator arm, said conducting transmission line having one ormore conducting dimples projecting through said actuator arm andpositioned facing said transmission line gap; and an arm electrostaticplate formed on said actuator arm, said arm electrostatic plate having afirst portion formed proximate said cantilever anchor and a secondportion extending from said first portion along said actuator arm,wherein when said switch is in an open position, said actuator arm has abend such that a minimum separation distance between said substratetransmission line and said one or more conducting dimples is equal to orgreater than a maximum separation distance between said substrateelectrostatic plate and said arm electrostatic plate, said armelectrostatic plate and a segment of said actuator arm on which said armelectrostatic plate is formed defining a structure electrostaticallyattractable toward said substrate electrostatic plate upon selectiveapplication of a voltage to said arm electrostatic plate.
 34. Themicro-electromechanical switch of claim 33, wherein the electrostaticattraction of said electrostatically attractable structure toward saidsubstrate electrostatic plate causes said one or more conducting dimplesto contact said substrate transmission line to bridge said gap.
 35. Themicro-electromechanical switch of claim 33, wherein said substratecomprises a semi-insulating substrate.
 36. The micro-electromechanicalRF switch of claim 35, wherein said semi-insulating substrate comprisesgallium arsenide (GaAs).
 37. The micro-electromechanical switch of claim33, wherein said actuator arm comprises polycrystalline silicon.
 38. Themicro-electromechanical switch of claim 33 wherein said bend in saidactuator arm is produced by inducing a nonuniform level of residualstress in said arm electrostatic plate formed on said actuator arm. 39.The micro-electromechanical switch of claim 33 wherein said bend in saidactuator arm is produced by inducing a nonuniform level of residualstress in said actuator arm.
 40. The micro electromechanical switch ofclaim 33, wherein said substrate electrostatic plate and saidtransmission line comprise gold microstrips on said substrate.
 41. Themicro-electromechanical switch of claim 33, wherein said one or moreconducting dimples comprise a metal selected from the group consistingof gold, platinum, and gold palladium.
 43. A method of fabricating anelectromechanical switch on a substrate comprising the steps of: a)forming a first layer of electrically conductive material over a surfaceof said substrate, said first layer comprising an arm plate contactregion, a substrate electrostatic plate region, an input transmissionline region, and an output transmission line region, wherein each ofsaid regions is electrically isolated from other said regions; b)depositing a sacrificial layer over said first layer of electricallyconductive material; c) forming an arm structural layer, said armstructural layer providing a cantilever with a proximate end and adistal end, said distal end located above said input transmission lineregion and above said output transmission line region, said cantileverhaving a cantilever anchor and a cantilever arm, said cantilever anchorformed at said proximate end and located above said arm plate contactregion, said cantilever arm projecting from said cantilever anchor tosaid distal end and located above said substrate electrostatic plateregion; d) removing a portion of said arm structural layer at saiddistal end to create a dimple area above said input transmission lineregion and said output transmission line region; e) removing a portionof said arm structural layer at said cantilever anchor to expose saidarm plate contact region; f) depositing a layer of metal on said armstructural layer, said layer of metal filling said dimple area and saidlayer of metal providing an arm electrostatic plate electricallyconnected to said arm plate contact region, said arm electrostatic platelocated generally above said substrate electrostatic plate region; andg) removing said sacrificial layer, wherein the step of forming the armstructural layer is performed to induce stress in the arm structurallayer that varies from the stress of the layer of metal deposited on thearm structural layer so that the cantilever arm will bow upwards whenthe sacrificial layer is removed.
 44. The method of claim 43, whereinsaid sacrificial layer comprises a layer of silicon dioxide.
 45. Themethod of claim 43 wherein said arm structural layer comprises siliconnitride.
 46. The method of claim 43, wherein said step of depositing alayer of metal on said arm support layer comprises the steps of: sputterdepositing a thin film of titanium; and depositing a layer of gold. 47.The method of claim 43, wherein the step of removing the sacrificiallayer is performed by wet etching the sacrificial layer.
 48. Amicro-electromechanical switch formed on a substrate comprising: asubstrate transmission line formed on said substrate, said substratetransmission line having a gap forming an open circuit; a substrateelectrostatic plate formed on said substrate; and an actuating portion,said actuating portion comprising: a cantilever anchor formed on saidsubstrate; a cantilevered actuator arm having a proximate end and adistal end, said cantilevered actuator arm attached to the cantileveranchor at the proximate end of the cantilevered actuator arm acantilever insulating layer attached at the distal end of thecantilevered actuator arm; an electrical contact formed on saidcantilever insulating layer and positioned facing said transmission linegap; and an arm electrostatic plate formed on said actuator arm, saidarm electrostatic plate having a first portion formed proximate saidcantilever anchor and a second portion extending from said first portionalong said actuator arm, wherein when said switch is an open position,said actuator arm has a bend such that a minimum separation distancebetween said transmission line and said electrical contact is equal toor greater than a maximum separation distance between said substrateelectrostatic plate and said arm electrostatic plate, said armelectrostatic plate and a segment of said actuator arm on which said armelectrostatic plate is formed defining a structure electrostaticallyattractable toward said substrate electrostatic plate upon selectiveapplication of a voltage to said arm electrostatic plate.
 49. Themicro-electromechanical switch of claim 48, said switch furthercomprising one or more mechanical stops formed on said substrate, saidmechanical stops disposed adjacent said substrate electrostatic plateand having a height greater than a height of the substrate electrostaticplate
 50. The micro electromechanical switch of claim 48, wherein theelectrostatic attraction of said electrostatically attractable structuretoward said substrate electrostatic plate causes said electrical contacton said actuator arm to bridge said transmission line gap.
 51. The microelectromechanical switch of claim 48, wherein said substrate comprises asilicon substrate with a layer of silicon nitride.
 52. The microelectromechanical switch of claim 48, wherein said actuator armcomprises polycrystalline silicon.
 53. The micro electromechanicalswitch of claim 48 wherein said bend in said actuator arm is produced byinducing a nonuniform level of residual stress in said arm electrostaticplate formed on said actuator arm.
 54. The micro-electromechanicalswitch of claim 48 wherein said bend in said actuator arm is produced byinducing a nonuniform level of residual stress in said actuator arm. 55.The micro-electromechanical switch of claim 48, wherein said substrateelectrostatic plate comprises polysilicon.
 56. Themicro-electromechanical switch of claim 48, wherein said electricalcontact comprises a metal selected from the group consisting of gold,platinum, and gold palladium.
 57. The micro-electromechanical switch ofclaim 48, wherein said arm electrostatic plate comprises a metalselected from the group consisting of gold, platinum, and goldpalladium.
 58. The micro electromechanical switch of claim 48, whereinsaid cantilever insulating layer comprises silicon nitride.
 59. A methodof fabricating an electromechanical switch on a substrate comprising thesteps of: a) depositing a first layer of electrically conductivematerial over a surface of said substrate; b) depositing a sacrificialcantilever support layer over said first layer of electricallyconductive material; c) forming a arm structural layer of electricallyconductive material over said sacrificial cantilever support layer, saidarm structural layer having a proximate end and a distal end, said armstructural layer having a cantilever anchor disposed at said proximateend, said cantilever anchor formed on said surface of said substrate,and said arm structural layer having a cantilever arm projecting fromsaid cantilever anchor; d) depositing a first metal layer on saidcantilever arm, said first metal layer located generally above saidfirst layer of electrically conductive material; e) depositing a secondmetal layer on or above said substrate to form a transmission line witha gap in the middle, said second metal layer located adjacent saiddistal end; g) depositing a conductor sacrificial layer on top of saidsecond metal layer; h) depositing a third metal layer on top of saidconductor sacrificial layer, said third metal layer positioned abovesaid second metal layer and extending across said gap; i) forming aninsulating layer above said third metal layer and located adjacent saiddistal end, said insulating layer attaching to said distal end and tosaid third metal layer; and j) removing said sacrificial layers, whereinthe step of forming the arm structural layer is performed to inducestress in the arm structural layer that varies from the stress of thefirst layer of metal so that the cantilever arm will bow upwards whensaid sacrificial layers are removed.
 60. The method of claim 59 furthercomprising the step of: forming a mechanical stop layer over saidsurface of said substrate, said mechanical stop layer disposed proximateto said first layer of electrically conductive material and having aheight greater than a height of said first layer of electricallyconductive material, wherein the step of forming a mechanical stop layerbeing performed prior to the step of depositing a sacrificial cantileversupport layer.
 61. The method of claim 59, wherein said first layer ofelectrically conductive material comprises polysilicon.
 62. The methodof claim 60, wherein said step of forming a mechanical stop layercomprises the steps of: depositing a mechanical stop sacrificial layeron top of said first layer of electrically conductive material; etchingsaid mechanical stop sacrificial layer to form areas for the mechanicalstops; and filling the areas for the mechanical stops with polysiliconto form said mechanical stops.
 63. The method of claim 59, wherein saidarm structural layer comprises polysilicon.
 64. The method of claim 59,wherein said insulating layer comprises silicon nitride.